A novel chemolithotrophic metabolism based on a mixed arsenic-sulfur species has been discovered for the anaerobic deltaproteobacterium, strain MLMS-1, a haloalkaliphile isolated from Mono Lake, California, USA. Strain MLMS-1 is the first reported obligate arsenate-respiring chemoautotroph which grows by coupling arsenate reduction to arsenite with the oxidation of sulfide to sulfate. In that pathway the formation of a mixed arsenic-sulfur species was reported. That species was assumed to be monothioarsenite ([H₂As^IIIS^-IIO₂]^-), formed as an intermediate by abiotic reaction of arsenite with sulfide. We now report that this species is monothioarsenate ([HAs^VS^-IIO₃]^2-) as revealed by X-ray absorption spectroscopy. Monothioarsenate forms by abiotic reaction of arsenite with zero-valent sulfur. Monothioarsenate is kinetically stable under a wide range of pH and redox conditions. However, it was metabolized rapidly by strain MLMS-1 when incubated with arsenate. Incubations using monothioarsenate confirmed that strain MLMS-1 was able to grow (µ = 0.017 h^-1) on this substrate via a disproportionation reaction by oxidizing the thio-group-sulfur (S^-II) to zero-valent sulfur or sulfate while concurrently reducing the central arsenic atom (As^V) to arsenite. Monothioarsenate disproportionation could be widespread in nature beyond the already studied arsenic and sulfide rich hot springs and soda lakes where it was discovered.